Organic Compounds: Hydrocarbons

Submitted by ChemPRIME Staff on Thu, 12/16/2010 - 12:56

Carbon is unique among the elements of the periodic tableA chart showing the symbols of the elements arranged in order by atomic number and having chemically related elements appearing in columns. because of the ability of its atomsThe smallest particle of an element that can be involved in chemical combination with another element; an atom consists of protons and neutrons in a tiny, very dense nucleus, surrounded by electrons, which occupy most of its volume. to form strong bonds with one another while still having one or more valences left over to link to other atoms. The strength of the carbon-carbon bond permits long chains to form:

Image:General Hydrocarbon Structure.jpg

This behavior is referred to as catenation. Such a chain contains numerous sites to which other atoms (or more carbon atoms) can bond, leading to a great variety of carbon compounds, or organicRefers to the branch of chemistry that studies compounds containing carbon, usually in combination with hydrogen and other elements such as O, N, S, and P. Certain small ions and compounds containing carbon (such as carbonate ions and carbon dioxide) are not considered to be organic, but rather are classed as inorganic. compounds. The hydrocarbons contain only hydrogen and carbon. They provide the simplest examples of how catenation, combined with carbon’s valence of 4, gives rise to a tremendous variety of molecular structures, even with only two elements involved. Single bonded hydrocarbons are called alkanes. A example of an alkane is butane:


These hydrocarbons can be in straight chains of varying length, or they can branch out, with one carbon bonded to three or four other carbons. This allows for isomers, such as iso-butane, a branched hydrocarbon:


Hydrocarbons can also form ring structures, which are referred to as cycloalkanes. An example is cyclohexane:


Carbons are capable of forming double and triple bonds with other carbons. This leads to molecules called alkenes, which contain a double bondAttraction between two atoms (nuclei and core electrons) that results from sharing two pairs of electrons between the atoms; a bond with bond order = 2., and alkynes, which contain a triple bond. An example of an alkene is ethene (ethylene), and an example of a alkyne is ethyne (acetylene):


Together, they are referred to as unsaturated hydrocarbons, since there are fewer hydrogen atoms in the molecule due to multiple bonds as compared to alkanes. A special class of multiple bond hydrocarbons are the aromatic hydrocarbons, which all take the form of hydrocarbon ring structures with double bonds between the carbons. Benzene is an example:


The hydrocarbons are also extremely important from an economic and geopolitical point of view. The fossil fuels, coal, petroleum (or crude oil), and natural gasA state of matter in which a substance occupies the full volume of its container and changes shape to match the shape of the container. In a gas the distance between particles is much greater than the diameters of the particles themselves; hence the distances between particles can change as necessary so that the matter uniformly occupies its container. consist primarily of hydrocarbons and are extremely important in everyday life. Petroleum turns out to be a mixtureA combination of two or more substances in which the substances retain their chemical identity. of many different hydrocarbons. Molecules of different sizes are useful for different tasks. The following schematic of petroleum fractional distillation shows the different types of hydrocarbons fractions taken from petroleum.

Figure 1 A schematic of the fractional distillation of crude oil used in petroleum refining. The mixture is separated into Gases, Gasoline, Kerosene, Fuel oil, Lubricating oil, and Residue(asphalt).

Following is a short overview of the the different fractions from petroleum[1] .

(Natural) Gases

The gas fraction contains hydrocarbons containing 1 to 4 carbon atoms in each molecule. These can be used for fuels. Another use is to derive materials such as plastics and syntheticDescribes a substance that has been manufactured‐one that has not been created by natural processes. fibers from such hydrocarbons, accomplished by polymerization techniques. An example is given below, propane:



Probably the most familiar of the hydrocarbon distillates is gasoline. Gasoline consists of hydrocarbons with 5 to 12 carbon atoms in each molecule. It is difficult to overstate the importance of gasoline to modern society, given the central role of automobile travel in our society. Gasoline also serves as an industrial solventThe substance to which a solute is added to make a solution.. An example of a hydrocarbon found in gasoline is toluene:



Kerosene consists of hydrocarbons containing between 12 and 16 carbon atoms per molecule. The foremost uses of kerosene are as lamp oil, diesel fuel, and for catalytic cracking, a processes discussed in the section on unsaturated hydrocarbons. This allows these larger hydrocarbons to be broken down to a size that can be used for gasoline. An example of a hydrocarbon that would be in the kerosene fraction is tetradecane:


Fuel Oil

Fuel Oils consist of hydrocarbons ranging between 15 and 18 carbon atoms per molecule. Like kerosene, this distillate is used for heating oil, for diesel fuel, and for catalytic cracking. An example is hexadecane:


Lubricating Oil

Lubricating oils consist of 16 to 20 carbon atoms per hydrocarbon molecule. Referred to sometimes as mineral oil, lubricating oils are used to decrease friction between moving parts. Perhaps the most familiar application is motor oil. An example of a hydrocarbon in the size range for lubricating oil is eicosane:



Hydrocarbons which are not boiled away remain after the distillation as hydrocarbons with more than 20 carbon atoms per molecule. These hydrocarbons can be used as asphalt. An example is tetracosane:


  1. Moore, J.W.; Stanitski, C.L.; Jurs, P.C. Chemisty:The Molecular Science. 3rd edition. Thompson Brooks/Cole. 2008. 546-547.